Introducing multiple luminescent centers into colloidal nanocrystals is an attractive way to impart new optical properties into this class of materials. Doping disparate ions into specific nanocrystals is often challenging, due to the preferential incorporation of one type of dopant. Here, we demonstrate the coexistence of europium dopants as divalent and trivalent ions in colloidal Ga 2 O 3 nanocrystals, achieved by controlled in situ reduction of Eu 3+ to Eu 2+ . The two dopant species exhibit distinctly different steady-state and time-resolved photoluminescence, and their ratio can be modified via doping concentration, reaction temperature, or thermal treatment of as-synthesized NCs. The Eu 2+ ions are proposed to be stabilized internally owing to the attractive interaction with oxygen vacancies, while Eu 3+ dopants partly reside in the nanocrystal surface region. The relationship between the electronic structure of the native defects and the dopant centers is discussed in the context of the overall emission properties. The exposure of these samples to X-ray radiation leads to the reduction of Eu 3+ to Eu 2+ , demonstrating an alternative way of manipulating the oxidation state and suggesting the potential application of this material as an X-ray storage phosphor. The coexistence of Eu 2+ and Eu 3+ and the ability to control their relative fraction over the full oxidation state range in group III oxide nanocrystals allow for the design and preparation of new photonic and light emitting materials.

Semiconductor photocatalysis has emerged as an efficient and sustainable advanced oxidation process for wastewater treatment and other environmental remediation and forms the basis for water splitting and solar-to-fuel conversion. Nanocrystalline metal oxides are particularly promising photocatalysts because of their efficiency, stability, and low toxicity. However, the influence of the crystal structure and defects on the photocatalytic activity of these polymorphic materials is still poorly understood. In this work we investigated the structural dependence of the photocatalytic activity of nanocrystalline Ga2O3. We demonstrate that metastable cubic-phase γ-Ga2O3 prepared from colloidal nanocrystals exhibits an anomalously high photocatalytic activity, which rapidly decreases upon thermally induced transformation to monoclinic β-Ga2O3. Using steady-state and time-resolved photoluminescence measurements we showed that the reduction in photocatalytic activity upon annealing is accompanied by a decrease in native defect (i.e., oxygen vacancy) concentration and interactions. Trapping charge carriers in defect-induced states in γ-Ga2O3 nanocrystals results in a reduced rate of charge recombination and enhanced interfacial charge transfer, which has been unambiguously confirmed by comparative measurements using In3+-doped Ga2O3. These phenomena are enabled by the unique character of defect states in γ-Ga2O3 nanocrystals which have much longer lifetime than typical metal oxide surface states. Using various scavengers, we demonstrated that reactive radicals (OH• and O2 •–) formed by photogenerated charge carriers play a key role in the mechanism of photocatalytic degradation by Ga2O3. The results of this work demonstrate how manipulation of the location and electronic structure of defect sites in nanostructured metal oxides can be effectively used to control charge carrier separation and enhance photocatalytic activity, without a detriment to high surface-to-volume ratio.

Investigation of the origin of high-Curie temperature ferromagnetism in diluted magnetic oxides has become one of the focal points of research on solid-state magnetism. While several possible mechanisms have been proposed theoretically, broader experimental evidence is still lacking. Here we report a comparative study of the electronic structure and magnetic properties of colloidal Fe-doped In2O3 and SnO2 nanocrystals, as building blocks for grain-boundary-rich diluted magnetic oxide films. The dopant ions in both nanocrystal host lattices are principally in 3+ oxidation state, with possibly a minor presence of Fe2+ in In2O3, and no conclusive evidence of the presence of Fe2+ in SnO2 nanocrystals. Subsequently, we found that Fe-doped In2O3 nanocrystalline films exhibit only minor ferromagnetic ordering (with a magnetic moment of less than ca. 0.1 μB/Fe) and decreasing saturation magnetization with increasing doping concentration at room temperature. The saturation magnetic moment of Fe-doped SnO2 nanocrystalline films is insignificant or below the detection limit. These results contrast previous findings for analogous Mn-doped nanocrystals, which contain mixed oxidation states (Mn2+ and Mn3+) and exhibit a robust ferromagnetism at room temperature. The correlation between the mixed dopant oxidation states and the observed magnetic properties implies that ferromagnetism in these systems is of a Stoner type, enabled by electron transfer between dopant ions and the local defect states arising from the grain boundaries within a nanocrystalline film. These results suggest the prospect of probing and manipulating ferromagnetism in nonmagnetic oxides by simultaneous control of the transition metal dopant oxidation states and extended structural defects.

We report the synthesis of ternary gallium tin oxide nanocrystals, and demonstrate that their photoluminescence can be tuned through the visible region by changing Ga:Sn ratio. By substitutional doping with Ga 3+ , the PL intensity of SnO 2 nanocrystals is enhanced by nearly 3 orders of magnitude, reaching photoluminescence quantum yield of >40%. Increase in PL intensity is attributed to the formation of donor and acceptor pairs, and the increase in emission energy is discussed in the context of band-gap expansion and stronger Coulomb interaction between charged defect sites. Time-resolved and steady-state photoluminescence spectroscopies reveal that the interaction of extrinsic and native defects is driven by the nature of the dopant ion. By adjusting various reaction conditions, we prepared the nanocrystals with nearly ideal scotopic-tophotopic ratio and a quantum yield of ca. 34%, attesting to the potential of these nanocrystals for general lighting applications. The results of this work provide new insight into the role of defect chemistry in tailoring the optoelectronic properties of transparent metal oxide nanocrystals, and pave the way for the rational design of light sources and photonic devices with high photoluminescence efficiency, minimum toxicity, and optimal lighting characteristics.

We demonstrate the coexistence of Eu(2+) and Eu(3+) in corundum and bixbyite-type colloidal In2O3 nanocrystals. The emission properties of dopants in both oxidation states are determined by their interaction with native defects, and are dramatically different in the two nanocrystal phases. This difference arises from the smaller nanocrystal size and higher defect density in metastable corundum-type nanocrystals.